1. Field of the Invention
The present invention relates to flame retardant polyurethane compositions, and more particularly to flame retardant flexible polyurethane foam compositions, methods of flame retarding flexible polyurethane foam compositions, articles made therefrom and flame retardants that comprise blends of alkylated triaryl phosphate esters and phosphorus-containing flame retardant additives.
2. Description of Related Art
Polyurethanes are polymers produced by the reaction of an isocyanate and a hydroxyl-containing material, such as a polyol. Polyurethanes are available in many different forms including a flexible foam, which is typically formed by treating a polyol with a diisocyanate in the presence of water and a catalyst. This reaction results in a flexible foam composition having an abundance of carbon-hydrogen bonds, a large surface area, and an open cell structure. Due, at least in part, to these characteristics, the flexible polyurethane foam is extremely susceptible to combustion. Accordingly, flame retardant additives are often used to reduce the risk and severity of flexible polyurethane foam combustion.
Flame retardant additives commonly used in flexible polyurethane foams typically contain halogens. Non-halogen flame retardant additives have also been used. However, non-halogen flame retardant additives may be less efficient than halogenated flame retardants, thereby prompting the use of higher flame retardant additive levels to compensate for this lower efficiency. Unfortunately, the use of higher flame retardant additive levels may contribute to reduced foam physical properties.
Triaryl phosphate-based flame retardant agents have been used in thermoplastic compositions. However, such agents may form a liquid deposit on the injection molds, which may adversely affect the environmental stress crack resistance of the resulting thermoplastic resins.
Historically, the first commercially available triaryl phosphates were derived from coal tar and included tricresyl phosphate, trixylyl phosphate, and mixtures of triaryl phosphates. Isopropylated and butylated synthetic triaryl phosphates were later developed as direct substitutes for coal tar derivatives and were mainly used as plasticizers for polyvinylchloride (PVC) compositions. During the development of flame retardant applications, the trend for newer additives was toward higher performance through increased phosphorus content and lower viscosity.
Alkylated triaryl phosphate esters have been used as flame retardants for flexible polyurethane foam for many years. These esters combine good hydrolytic and thermal stability with flame retardant efficiency provided by their phosphorus content. Triaryl phosphate esters are used either alone, or more commonly, in conjunction with halogenated additives.
U.S. Pat. No. 4,746,682 to Green discloses blending alkylated triaryl phosphate esters with brominated diphenyl oxide for use as a flame retardant for polyurethane. U.S. Pat. No. 4,565,833 to Buszard et al. discloses the use of isopropylated phosphate esters as flame retardant agents in rigid polyurethanes or polyisocyanurates. These esters contain varying levels of triphenyl phosphate (TPP). The use of triphenyl phosphate has hereto been desirable because it was found to reduce viscosity and contribute to higher phosphorus content. Phosphorus is known to contribute to performance as a flame retardant.
Alkylated triphenyl phosphates that have been recommended and used as flame retardants have had phosphorus contents in excess of 7.7 percent. Butylated phosphates available and recommended for use as flame retardants have a phosphorus content of at least 8 percent (AKZO-Nobel Functional Chemicals Bulletin 99-94, 1999).
U.S. Pat. No. 5,958,993 to Blundell et al. discloses the use of resorcinol bisdiphenylphosphate (RDP) to impart flame retardency to polyurethane compositions.